Technical Field
The present invention relates to optical fiber systems and, in particular, to a duo-functionality optical fiber sensor system.
Description of the Related Art
Optical fiber technology is ideal for telecommunication systems for its low-loss, high-bandwidth, low-dispersion advantages, as well as its immunity to Electromagnetic Interference (EMI). Optical components, such as optical isolators and optical diodes, are usually applied in fiber-optic communication systems and allow the transmission of light in only the forward direction. This is done to prevent unwanted feedback into the optical oscillator. Additionally, an optical fiber sensor system is another significant application of optical fiber technology thanks to its low-cost, its superior sensitivity, its light-weight technology, its electrical-safety, its remote-access, and its ease of being able to be multiplexed.
So far, numerous distributed sensors using Single-Mode Fibers (SMFs) have been applied in a variety of important fields, such as structural sensing, smart structures, and civil engineering; aerospace and security; marine, oil and gas; and health monitoring. Such fiber optic sensor technology, such as Optical Time Domain Reflectometry (OTDR), mainly depends on measuring the frequency shift or power intensity of Brillouin or Raman scattering light in only the backward direction. Meanwhile, passive components, such as optic terminators, are applied to absorb light coming from the unwanted forward direction. Nonetheless, the optical couplers (optical isolators and optic terminators) in both cases reduce the optical power of forward signal or back reflection, wasting the precious carried information along with it. Little research has been done on the sensor systems that can simultaneously take full advantage of both forward and backward propagating signals in the fiber with high spectral efficiency and low coupling loss.
For particular applications, fiber optic sensors are likely deployed in various harsh environments, such as in dry deserts, on the ocean floor for seismic sensing, or in downhole environments for oil and gas exploration and production. This may create big challenges in the establishment of a long-term, high-speed, and stable communication method between sensing edges and receiver terminals. The implementation of an extra special communication cable will be time consuming, costly, and even technically impossible, for some cases. Moreover, this could add additional maintenance costs for the entire sensing system. So far, there is no mature research that can achieve such duo-functionality, such as signal transmission and distributed sensing, simultaneously within the same fiber. This simultaneous signal transmission and distributed sensing would be of great importance for sensing in sophisticated target environments.
Another main challenge related to optical fiber sensor systems is how to measure and separate a large number of different measurands, which is a challenge that requires the use of many electronic point sensors. The most common functions are temperature and strain/stress sensing. However, a variety of other parameters, such as pressure, magnetic field, rotation and acceleration, voltage, and chemical species might also be measured. So far, one particular type of sensor typically corresponds to one measurand, but there are exceptions. SMFs, the Brillouin frequency shift, and the Brillouin power change were measured to achieve concurrent temperature and strain sensing, but the spatial resolution was too poor. Therefore, there is a need for new optical fiber sensor systems capable of both highly enhanced spatial resolution and accurate, simultaneous measurements of a large number of different measurands.